Rubber compositions for tire components, such as for example tire treads, are often comprised of at least one diene-based elastomer and reinforcing filler reinforcement comprised of rubber reinforcing carbon black and precipitated silica. The rubber composition usually contains a silica coupler to aid in coupling the precipitated silica to the diene-based elastomer(s).
Such rubber compositions may further contain short, discontinuous fiber reinforcement. Such short, discontinuous fiber reinforcement may comprise, for example, fibrillated aramid fibers comprised of a trunk portion containing numerous short fibrils extending outwardly from its trunk having diameters substantially smaller than its trunk diameter. Exemplary of such fibrillated aramid are various Kevlar® fibers (e.g. Kevlar® aramid pulp) from the E. I. DuPont Company.
Because of the fibrillated configuration of the short aramid fibers, such fibrillated aramid fibers have been observed to tend to clump together in a form of a pulp which is difficult to adequately disperse within a rubber composition without forming a plurality of domains comprised of clumps of the fibrillated aramid fiber pulp within the rubber composition.
Such fibrillated aramid fiber pulp has been described in U.S. Pat. No. 4,871,004 (which is incorporated herein in its entirety) as being comprised of aramid fibers composed of a trunk portion having a length of from about 0.2 to about 5 mm (micrometers), an aspect ratio of greater than 100 containing numerous fibrils extending outwardly from its trunk portion with diameters substantially smaller than its trunk diameter and having a surface area of from about 4 to about 20 square meters per gram.
Fibrillated aramid micropulp has been described in U.S. Patent Publication No. 2003/0114641 (which is incorporated herein in its entirety) as fibrillated aramid fiber pulp treated (referred to herein as being micronized to fracture its trunk portion and to thereby reduce the average length of its trunk portion) to form a fibrillated aramid fiber micropulp thereof to provide fibrillated aramid fiber micropulp intended to be more readily dispersible in a rubber composition to thereby promote better dispersion of the micropulp within a rubber composition and more adequate fiber reinforcement of the rubber composition.
It is envisioned herein that such fibrillated aramid fiber micropulp may be comprised of micronized fibrillated aramid fibers having a trunk portion reduced in length by a factor, for example, in a range of from about 4 to about 50 times from its aforesaid fibrillated aramid fiber length and a surface area increased by a factor, for example, in a range of from about 10 to about 100 times the surface area of its aforesaid fibrillated aramid fiber.
For this invention, it is desired to provide a rubber composition comprised of at least one diene-based elastomer with a combination of such fibrillated aramid fiber micropulp rubber reinforcement with rubber reinforcing filler comprised of pre-hydrophobated precipitated silica.
For such pre-hydrophobated precipitated silica it is proposed to provide a precipitated silica as a pre-hydrophobated precipitated silica (hydrophobated prior to its addition to the rubber composition) as a precipitated silica pre-treated with at least one of bis(3-alkoxysilylalkyl) polysulfide and alkoxyorganomercaptosilane. Such pre-treatment may optionally also include an alkylsilane.
Because the precipitated silica is generally hydrophilic in nature, thereby tending to clump together within the rubber composition and become challenging to adequately disperse within the rubber composition, such pre-treatment of the precipitated silica may be desirable to thereby pre-hydrophobate the precipitated silica and hereby enable it to more readily disperse within the rubber composition.
One intended purpose of providing said fibrillated aramid micropulp reinforcement together with the pre-hydrophobated precipitated silica filler reinforcement is to increase the storage modulus (G′) at low strain of the sulfur cured rubber composition containing the pre-hydrophobated precipitated silica desirably without significantly increasing the hysteresis of the cured rubber composition as evidenced by not significantly changing its tan delta physical property.
In the description of this invention, the terms “compounded” rubber compositions and “compounds”, where used, refer to the respective rubber compositions which have been compounded with appropriate compounding ingredients. The terms “rubber” and “elastomer” may be used interchangeably unless otherwise indicated. The amounts of materials are usually expressed in parts of material per 100 parts of rubber by weight (phr).